Problem: The geometric sequence $(a_i)$ is defined by the formula: $a_i = -1 \left(2\right)^{i - 1}$ What is $a_{4}$, the fourth term in the sequence?
Explanation: From the given formula, we can see that the first term of the sequence is $-1$ and the common ratio is $2$ To find $a_{4}$ , we can simply substitute $i = 4$ into the given formula. Therefore, the fourth term is equal to $a_{4} = -1 \left(2\right)^{4 - 1} = -8$.